Midline versus bilateral miniscrews with clear aligners for intrusion of maxillary anteriors: A finite element model analysis

INTRODUCTION

Deep bite is a frequently observed malocclusion in orthodontic practice, with a global prevalence of 21.98% and variation from 8.4% to 51.5%.^[1,2] It is characterized by maxillary incisors overlapping mandibular incisors by over 30–40% and results due to the interplay of factors such as jaw growth, soft tissue function, jaw musculature, and dentoalveolar development.^[3,4] Clinically, deep bite manifests with increased gingival display, significant dental attrition, labial or lingual inclination of incisors, mandibular displacement, hyperactivity of the masticatory muscles, temporomandibular joint disorders, and esthetic concerns.^[5,6] Treatment plans are formulated based on malocclusion severity, skeletal factors, growth patterns, and individual patient esthetic considerations. Corrective measures typically involve procedures such as molar extrusion, incisor intrusion, or a combination of both.^[7-9]

Temporary anchorage devices (TADs) or miniscrews provide a contemporary solution for anterior intrusion. They are effective for direct intrusion while maintaining anchorage.^[8]Bilateral TADs or miniscrews are effective for obtaining true intrusion, as the line of force passes through the center of resistance (Cres) as well as effective in achieving asymmetric intrusion,^[10-12] whereas midline miniscrews are documented to cause proclination as they are positioned away from Cres^[13,14] and encounter the challenge of insertion due to presence of heavy labial frenum.^[8-12]

The rise in adult orthodontic patients demands esthetic alternatives. Clear aligners, which offer esthetic appeal, comfort, better dental hygiene, and less pain, can treat complex malocclusions.^[15] Despite their benefits, there is substantial disagreement on the efficacy and efficiency of clear aligners in controlling orthodontic tooth movement (OTM).^[16,17] Precision for mandibular and maxillary central incisors intrusion is 47% and 45%, respectively, while maxillary lateral incisors show the least intrusion precision of 33%.^[16-19] Even so, correction of deep bite with clear aligners is less predictable and challenging compared to fixed appliances.^[16,17,19] Additional biomechanical enhancement by incorporating elastics along miniscrew anchorage, along with clear aligners, can facilitate anterior intrusion and effectively correct deep bites and gummy smiles.^[20,21]

The utilization of three-dimensional finite element model (FEM) analysis offers a precise and non-invasive means of assessing tissue responses within critical structures such as the periodontal ligament (PDL), teeth, and alveolar bone using simulated force scenarios that provide valuable insights unattainable through direct human experimentation.^[24] The placement of midline versus two miniscrews bilaterally for intrusion purposes can lead to varying amounts and patterns of stress on the periodontium. While utilizing two mini-screws may yield better overbite correction, it is important to acknowledge the potential deleterious effects of reciprocal stresses on surrounding tissues. Therefore, when deciding on the number of miniscrews for intrusion, careful consideration of both treatment efficacy and potential tissue consequences is warranted.

This FEM study aims to evaluate and compare stress distribution patterns on the maxillary dentition, PDL, alveolar bone, and aligners when using midline versus bilateral miniscrews assisted maxillary anterior intrusion with clear aligner therapy.

MATERIAL AND METHODS

Model creation

A cone-beam computed tomography (CBCT) data of a 27-year-old female patient with Class II division 1 malocclusion and a 5 mm deep bite within a good craniofacial architecture and complete dentition were retrieved from the archives of the department. The CBCT had been taken earlier for treatment purposes. The patient gave written and verbal informed consent for access to their data. The study received ethical approval from the Research and Recognition Committee under the Faculty of Dentistry (EC/412/42/2022).

Every CBCT slice had a thickness of 0.15 mm. The CBCT was received in a Digital Imaging and Communications in Medicine (DICOM) format. For exacting the dentition and intraoral surface geometry, an intraoral scan was taken with a 3Shape TRIOS Scanner and saved in Stereolithography format. Using CATIA V5R7 software, this STL file was further used to create a three-dimensional (3D) CAD model. The CAD geometry was created based on the reference points from the CBCT image.

3D CAD geometry of the maxilla, consisting of cortical and cancellous bone with the entire maxillary dentition, was created. In accordance with published studies, the PDL was created to have a consistent thickness of 0.25 mm on the exterior of the root.^[25,26] Aligner was constructed over the model to complete the assembly. The uniform overall thickness of the aligner was kept at 0.5 mm, taken from the existing literature.^[25,27,28] Once the CAD model was ready, it was then imported into pre-processing software Hypermesh 2021 for meshing. [Figure 1] shows the CAD model, and [Table 1] shows nodes and elements for the model.

Table 1: Number of nodes and elements in FE mesh using hypermesh software.

Sr. No.	Components	FE assembly
		Number of elements	Number of nodes
1	Maxilla	253298	55602
2	Teeth	178761	42661
3	PDL	41929	11245
4	Aligner	52653	17762

PDL: Periodontal ligament, FE: Finite element

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Figure 1:

Virtually constructed CAD model with meshing and its components (a) maxillary dentition; (b) aligner; (c) maxillary dentition with bone; (d) complete assembly of aligner over dentition with bone and PDL.

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The model assembly was subjected to miniscrew insertion points as fixed pivots, and the loading conditions were applied. The position of the miniscrew was taken 5 mm above the alveolar crest.^[13,14] The dimensions and material properties of miniscrews were not contemplated as they were irrelevant to the study.

Two submodels were developed

M1: With a midline miniscrew in the interradicular region of central incisors. Elastic was pulled over the aligner from the miniscrew and inserted into the cut-outs on the aligners over the central incisors palatally.

M2: With two lateral miniscrews between the interradicular region of the lateral incisors and canines bilaterally. Elastics were pulled over the aligner from the miniscrew and inserted into the cut-outs on the aligners over the central incisor and canine palatally on both sides. [Figure 2] shows the two conditions and loading mechanics with submodels, M1 and M2.

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Figure 2:

Virtual sub models with applied loading conditions and directions. (a) Sub model M1 with one mini screw; (b) Sub model M2 with two mini screws.

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RESULTS

In this study, the stress distribution was evaluated using the von Mises stress criterion, with measurements reported in megapascals (MPa). A color scale was utilized to quantitatively assess the stress distribution in the aligner, teeth, PDL, and bone. The scale ranges from blue to red, where blue represents the least stress concentration over the region, whereas red signifies the highest stress concentration [Supplementary Figures].

As illustrated in [Graph 1a], on aligners, the maximum stress levels of 0.1377 MPa were observed in configuration M1 under a 300 g force loading, whereas the minimum stress of 0.0119 MPa was recorded in configuration M2 under a 50 g loading. The peak stress concentrations occurred in the incisal region of the aligners. M2 resulted in a more distributed stress pattern, thereby reducing overall stress values. In the context of maxillary dentition, M1 the stresses were primarily concentrated on the incisal edges of these teeth due to the placement of elastics, as depicted in [Graph 1b]. Under a 300 g load, the maximum stress concentration of 0.30 MPa was observed on the incisal edges of the central incisors with M2. Conversely, the minimum stress of 0.020 MPa was noted in configuration M1 under a 50 g force. The stress values in M1 were more than twice that in M2, with stresses localized to the central incisors in M1 and more diffusely distributed over the laterals and canines in M2.

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Graph 1:

Stress concentration with various loads on (a) aligners; (b) maxillary dentition; (c) periodontal ligament; (d) alveolar bone of both submodels M1 (1 TAD) and M2 (2 TADs). TAD: Temporary anchorage devices.

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[Graph 1c] provides insight into PDL stress distribution. The minimum stress concentration, 0.0009 MPa, was observed with a 50 g loading force with M1. The maximum stress, 0.0094 MPa, occurred under a 300 g load in configuration M2, which is approximately 10 times higher than the minimum value. Despite this, the stress values were minimal and likely insignificant in clinical terms. [Graph 1d] shows the stress distribution within the maxillary bone. The highest stress concentration of 0.1751 MPa was recorded under a 300 g force applied to M1, while the lowest stress concentration of 0.014 MPa was observed with M2.

[Graph 2] depicts the displacement of the maxillary teeth along all three axes (x, y, z) in micrometers. Vertically, along the y-axis, the displacement of all components exhibits a direct increase with the loading value in both models. In Graph 2a, the vertical displacement for M1 with a 50 g load is twice that of the M2 configuration. When considering sagittal movements, the displacement values along the z-axis for M2 are half those of M1. This indicates that the M2 configuration provides better control over proclination for the same loading magnitude compared to M1.

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Graph 2:

(a) Displacement of anteriors in M1 (1 TAD) and M2 (2 TADs) for different loading conditions; (b) Displacement of canine and incisors for different loads on M2. TAD: Temporary anchorage devices.

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DISCUSSION

Orthodontic treatment objectives include addressing deep bite issues, often through maxillary incisor intrusion for patients with increased gingival display and normal vertical dimension.^[7-9] Clear aligners, while favored for their esthetic appeal and comfort, may not be as effective as traditional braces for complex cases and may require extended treatment durations. Vertical movements with aligners are difficult to achieve.^[16,17,32] Few authors have reported that intrusion of the anterior region is difficult to achieve and unpredictable with clear aligners^[35,36] while others concluded intrusion to be one of the most predictable movements.^[5,37]

Al-balaa et al.^[32] in their study of the predictability of movements with aligners found that maxillary lateral incisors demonstrated the highest accuracy of 58.12%, followed by maxillary central incisors at 51.83% and maxillary canines at 48.95%. Kravitz et al. observed a mean accuracy of 41.3% for anterior intrusion, with an average attempted pure intrusion of 0.72 mm.^[19] In addition, Krieger et al. reported a low agreement of 14.3% between anticipated and actual measurements of anterior intrusion, highlighting challenges in achieving precise movements in the vertical plane.^[17] Achieving more than 1 mm of anterior intrusion is challenging with clear aligners alone^[18] often necessitating the use of attachments or auxiliaries to enhance treatment outcomes.^[30,31]

Geramy et al. studied the effectiveness of miniscrew placement with clear aligners for maxillary whole arch intrusion^[33] and noted that when miniscrews were positioned distal to the lateral incisor, there was greater intrusion of the incisors compared to the canines. They observed that placing mini-screws mesially led to greater intrusion of teeth located mesially to the miniscrew compared to those distally positioned. In agreement with this, the current study observed higher stresses along the incisal edges of incisors compared to those of canines in M2 configuration. This suggests that the placement of mini-screws influences the magnitude of intrusion, indicating potential benefits for deep bite correction when mini-screws are positioned anteriorly.^[33,34] In this FEM study, the miniscrew positions were determined based on Cres of the upper incisors, typically located 8–10 mm apically and 5–7 mm distally to the lateral incisors as reported in previous literature.^[13,14]

The von Mises stress on teeth increased exponentially with loading force for both the configurations [Graph 3]. In M1, stresses were concentrated around the central incisors (0.006 MPa for 50 g at apices), especially the disto-incisal edges (0.051 MPa for 50 g), due to the positioning of miniscrews and elastics [Supplementary Figures]. In M2, stresses are more evenly distributed across the central and lateral incisors (0.0045 MPa for 50 g at apices and 0.021 MPa for 50 g at the incisal edges). Thus, in cases where intrusion of only central incisors is required, it is advisable to prefer M1 configuration over M2. These results align with previous studies, confirming similar stress distribution patterns.^{[12,32,34,40]}

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Graph 3:

(a) Stress concentration with respect to loading force for M1; (b) Stress concentration with respect to loading force for M2. TAD: Temporary anchorage devices.

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Active tooth movement compresses the periodontium, leading to root resorption, which shows individual variability despite similar force applications.^[23] The stress from elastics over mini-screws inevitably leads to some degree of root resorption, primarily due to compression on the apices and PDL during OTM.^[22] External apical root resorption (EARR) is a recognized pathological consequence of orthodontic treatment, with intrusion causing the highest rates; however, manipulation of applied forces can help reduce or control EARR. Miniscrew positioning is pivotal, since variations in stress are linked to screw placement and elastic force and thus affecting bite and dentition. Placement differences impact force distribution, influencing treatment outcomes and complications such as root resorption and bite alterations. High intrusive forces are a suspected cause of root resorption due to stress concentration at the conical root ends.^[22,23] Variations in root morphology and PDL thickness can further alter stress distribution. The less mineralized cementum at the root apex is more prone to resorption.^[45] This study found higher stress in the cervical PDL region than at the apex [Supplementary Figures]. This contrasts with FEM studies^[8,12,40,41] using fixed appliances, where stress is typically higher in the apical region. The difference is attributed to the force delivery of the appliance: Fixed appliances apply force from the miniscrew to the wire, sparing the incisal surface from direct force. On the contrary, clear aligners used with miniscrews exert pressure over the incisal edges due to the attachment of elastics to the palatal cutout of the aligner, therefore reducing the chances of EARR when compared to fixed appliances.

This FEM study found that M2 produced lower apical stresses than M1. Previous research indicates that external pressure around 0.0047 MPa can stimulate OTM and EARR, while 0.016 MPa can cause PDL necrosis.^[42,46] In this study, M1 with a 300 g load caused 0.0677 MPa stress, while M2 caused 0.0272 MPa stress. With a 100 g load, stresses were 0.0237 MPa (M1) and 0.0092 MPa (M2). Stress induced in M1 at the apices of anterior roots is nearly equivalent to the stress from 3 times the load in M2, making M2 configuration a more favorable choice for periodontally and anatomically compromised dentition.

At a 50 g load, M2 configuration produced insufficient stress for OTM, while in the M1 configuration stress was below root resorption levels. At 100 g load, M2 generated optimal stress for OTM with minimal root resorption, whereas M1 resulted in higher, concentrated stresses around central incisor roots. Overall, bilateral miniscrews yield lower stress levels than midline.

While M2 configuration showed stress distribution in the canine with a 50 g load, M1 did not show any stresses on the canine even with a 300 g loading. Therefore, the cases that require intrusion of incisors along with canines, M2 configuration should be preferred. Canine displacement increased uniformly with load across all axes. The canines exhibited less intrusive effect, compared to incisors, which is in conjunction with other studies.^[8,33,34] This may be due to mesial miniscrew placement and higher force requirement for equivalent intrusion [Graph 2b]. Using 50 g force with two miniscrews, the apical stress on the canine was 0.0023 MPa, increasing to 0.0076 MPa at 150 g and 0.0136 MPa at 300 g load. Higher load is required to cause enough stress for OTM of canine in M2 configuration; therefore, it is imperative to consider the consequent increase of stresses on the incisors, which might lead to their root resorption. Hence, more than 150 g loading should be avoided.

When the line of force moves away from the Cres, the resultant moment increases proportionally with the distance. This study observed proclination, involving both intrusion and movement in the labiolingual direction, which intensifies stress on the cervical portion of the PDL [Graph 3]. This stress contributes to the remodeling of the alveolar bone cervical margin, where the highest stresses were observed. Near the apex of the PDL, bone bending may occur, resulting in lower stresses.

In conjunction with other studies,^[17,37-39] Yan et al.^[39] in their analysis described the effectiveness of aligners in proclination and intrusion of upper incisors in Class II division 2 subjects and predicted the incisor proclination and intrusion achieved to be 69.8% and 53.3%, respectively. In this FEM study, it was observed that the M1 configuration had higher movement in sagittal direction in the form of labial flaring, hence making M1 configuration a better choice in Class II Division 2 cases.

This study analyzed incisor movement in three axes [Graph 2a]. Along the vertical axis (Y), intrusive displacements increased exponentially with higher loads for both the cases; M1 showed greater vertical displacement than M2 due to differing stress patterns. In the sagittal axis (Z), there was proportionate labial movement of the crown with increased load. Stresses around the cervical margin of the PDL and cervical bone indicate that midline miniscrew application resulted in more labial movement of the incisal edges compared to using two miniscrews, showing higher stress in M1 than M2 [Graph 3a and 3b]. Incorporating negative torque in the incisors as an overcorrection can help reduce resultant labial flaring.

CONCLUSION

1. Stress distribution over the dentition, PDL, and bone was found to be better with two bilateral miniscrews than with a midline miniscrew.

2. Displacement in all directions increased proportionally with load.

3. If only central incisors are to be intruded, midline miniscrew was found to be preferable, but if canines are to be included with anteriors, bilateral miniscrews with increased force values should be used.

4. Proclination was observed more with one miniscrew. Midline miniscrew is better in cases requiring a combination of intrusion and proclination, provided the forces used are lighter.

5. According to this study, the advisable force loading for maxillary anterior intrusion using clear aligner therapy should not exceed 150 g while using midline miniscrew and 200 g with bilateral miniscrews.